1. Field of the Invention
The present invention relates to a document transport device for use in a facsimile apparatus, copying machine or the like having an optical reading assembly, and more particularly to a device for transporting at a specified speed a document having data recorded thereon for the reading assembly to read the data during transport.
2. Description of the Prior Art
Such document transport devices heretofore generally used have the construction shown in FIG. 2. The illustrated document transport device S has a transport path 3 defined by a lower guide plate 1 and an upper guide plate 2 opposed to the plate 1 and spaced apart therefrom by a predetermined clearance for transporting a document 4 through the clearance. The transport path 3 is provided with a first transport roller 5, a first pinch roller 6 for pressing the document 4 against the roller 5, a second pinch roller 7 and a second transport roller 8 for pressing the document 4 against the roller 7. These rollers are arranged toward the direction of transport of the document 4 in the order mentioned. Between the first transport roller 5 and the second pinch roller 7, the lower guide plate 1 is formed with an aperture 9 for passing light therethrough to provide a position for optically reading the document. The transport device S has an optical reading assembly which comprises a light source 10 for projecting light on the document 4 at the apertured reading position 9, reflecting mirrors 11, 12, 13 for deflecting the light projected by the light source 10 and reflected from the document 4, a lens 14 for collimating the deflected light, and a reading device 15, such as a charge-coupled device (CCD), for receiving the collimated light to read the data recorded on the document 4.
To enable the reading assembly to read the data accurately, the document 4 must be transported and tensioned at least when the document 4 passes the apertured reading position 9. For this purpose, the second pinch roller 7 disposed downstream from the apertured reading position 9 with respect to the transport direction is rotated at a higher speed than the first transport roller 5 upstream from the apertured reading position 9, with the first pinch roller 6 adapted to exert a higher pressure than the second transport roller 8, so as to transport the document 4 at a constant speed corresponding to the speed of rotation of the first transport roller 5, while causing the second pinch roller 7, higher than the first transport roller 5 in rotational speed, to tension the document 4 between the two rollers 5 and 7.
However, in the case where the document 4 is tensioned between the two rollers 5, 7 by rotating these rollers 5, 7 at different speeds and pressing the two rollers 6, 8 against the rollers 5, 7, respectively, at different pressures, the vibration of the motor (not shown) for driving the rollers 5, 7 is transmitted to these rollers 5, 7, invariably deflecting the document 4 at the portion thereof opposed to the apertured reading position 9 between the rollers 5, 7. Consequently, the reading assembly is unable to accurately read the data on the document 4. For example, when the document 4 bears a longitudinal straight line, there arises the problem that the line is read as a nonlinear zigzag image.